1. Field of the Invention
The present invention relates to a cellular radio communication system, a radio base station apparatus and a radio terminal unit, and more particularly to a cellular radio communication system, a radio base station apparatus and a radio terminal unit having a mechanism for relieving the influence of interference even in a boundary area between the radio base stations in which a plurality of radio base stations cooperate and the quality of signal may be possibly degraded due to interference of the signals sent by the plurality of radio base stations.
2. Description of the Related Art
1. Cellular Communication
In the mobile radio communication, a cellular system is generally used to enable a mobile terminal and a base station to communicate in a service area extending as a plane. In the cellular system, a plurality of base stations are scattered in the service area, and a planar cover area is established by connecting the areas (terminal communicable areas) covered by the base stations. Each base station sends a reference signal for enabling self-station to be recognized. The reference signal is designed to be unique in the region in terms of signal series to be sent, sending time or frequency, or a combination of the signal series, the time and the frequency. The terminal receives the unique reference signals sent by each base station, and grasps a radio state with the plurality of base stations adjacent to the self-station by measuring and comparing respective strengths. Those measurement results of the radio state are used to search for the base station with greater signal strength and providing a more excellent reception state (probably in the shortest propagation distance). If it is judged that the base station providing the most excellent reception state is changed from a currently connecting base station to the other adjacent base station, hand-over of switching a connection to the base station expected to implement more excellent reception state is performed to realize the cellular communication.
FIG. 1 shows a configuration view of a radio communication system.
A concept of the cellular communication will be described below again, using FIG. 1. In the cellular communication, a plurality of base stations (20, 21, and 22) exist as shown in FIG. 1. A terminal 1 makes a radio communication with the base station 20. Each base station is connected to a network apparatus 50 to secure a wire communication path. The network apparatus 50 connected to the plurality of base stations is IP connected via a packet switch device 40. In the figure, the terminal 1 is communicating with the base station 20 located in the shortest distance and capable of receiving an excellent signal. Each base station (20, 21, and 22) sends the reference signal of its own identification signal. The terminal 1 receives the reference signal sent by each base station, and measures its reception strength. The base station at which the reception strength of the reference signal is the strongest is determined as the base station located in the shortest distance. In the figure, a downstream line signal (communication from the base station to the terminal) 30 and an upstream line signal (communication from the terminal to the base station) 31 are illustrated. The base station 20 sends the downstream signal 30, the base station 21 sends a downstream signal 32 and the base station 22 sends a downstream signal 33. Since each base station sends the signal at the same frequency and at the same time, there is possibility that the downstream signals 30, 32 and 33 interfere with each other. The terminal 1 located on a cell edge (a cell boundary) receives the desired signal 30 from the base station 20, but concurrently receives the interference waves 32 and 33 from the other stations and is affected by them. A ratio of interference power and noise power to a desired signal power is called a Signal Interference and Noise Power (SINR). On the cell edge, the interference from other cells is stronger and becomes a dominant term of the denominator, whereby the SINR is degraded and it is often difficult to convey the information at high throughput.
2. Fractional Frequency Reuse (FFR)
As a method for reducing the interference on the cell edge, an FFR is well known (refer to JP-A-2009-21787, JP-A-2009-44397, 3GPP TS36.331, 6.3.2 Radio resource control information elements, Mobile WiMAX-Part1 A Technical Overview Performance Evaluation, 4.2 Fractional Frequency Reuse, IEEE 802.16m System Description Document (IEEE 802.16m-08/003r7) 20.1 Interference Mitigation using Fractional Frequency Reuse, and 3GPP TS36.213, 5.2 Downlink power allocation). The FFR is performed in a multiplex system intended for a wide band communication such as an Orthogonal Frequency Division Multiplex Access (OFDMA). The FFR grasps whether the terminal is “located on the cell edge” or “located in the cell center”, and imposes restrictions on an assigned frequency depending on the location. Also, a sending power is changed with the assigned frequency. The assignment is controlled so that the frequencies used by the terminals located on the cell edge may not be equal, whereby the interference given to the surrounding cells is controlled in a frequency domain.
FIG. 2 shows a frequency use method for three base stations adopting the FFR. There are three base stations 20, 21 and 22, in which a horizontal axis indicates the frequency. A vertical axis indicates a signal power sent at each frequency. In the three base stations, a frequency band 60 is sent by weak sending power from all the base stations. Since all the base stations send the signal at this frequency, a frequency reuse ratio is 1. In this case, it is also called a reuse 1. This frequency band 60 is assigned to the terminals located in the cell center (distributed near the base station). Since an object of use is the terminal located in the cell center, a propagation loss of the signal sent from the desired base station is small, even if the sending power is weak, whereby the signal is received with high power. Also, the interference caused by the adjacent base station follows a longer propagation distance than the desired wave, and has a greater propagation loss than the desired wave, with its influence less significant. Therefore, the excellent signal quality is easily obtained.
At frequencies 61, 62 and 63, each of the three base stations sends the signal only at the frequency designated by itself, and does not send the signal at the other frequencies. In the case where the repetitive use is 3 as shown in the figure, it is also called reuse 3. This frequency band is assigned to the terminal on the cell edge. The object of use is the terminal on the cell edge, which is more susceptible to the interference from the adjacent cell, but because the adjacent cell repetitively uses the three different frequencies, or has the reuse 3, as described above, the influence of the interference wave is smaller.
FIG. 3 shows one example of the cellular communication composed of a plurality of cells. In this example, six base stations having the reference numerals 20, 21, 22, 23, 24 and 25 are illustrated. The base station 20 covers areas 100 and 101. The frequency 60 as shown in FIG. 2 is assigned to the terminal located in the area 100 in the cell center. A frequency 61 is assigned to the terminal located in the area 101. Also, in the adjacent base station 21, the frequency 60 as shown in FIG. 2 is assigned to the terminal located in an area 110 in the cell center. A frequency 62 is assigned to the terminal located in an area 111. Similarly, in the adjacent base station 22, the frequency 60 as shown in FIG. 2 is assigned to the terminal located in an area 120 in the cell center. A frequency 63 is assigned to the terminal located in an area 121.
On the boundaries of the areas 101, 111 and 121, the frequency 61 is used in the area 101, the frequency 62 is used in the area 111, and the frequency 63 is used in the area 121, whereby the same frequency is not used between the adjacent base stations. Accordingly, the influence of interference is greatly reduced.
3. Fractional Transmission Power Control (FTPC)
In the OFDMA, the frequency is divided into strips called a sub-carrier, using the FFT. Each base station enables a specific terminal to occupy a sub-channel (or also called a resource block) collecting a plurality of sub-carriers by scheduling in making the communication. Therefore, among the terminals belonging to the same cell, only one terminal can use a certain frequency (or sub-channel or resource block), whereby the interference using the same sub-channel in principle does not occur. This is a difference from a Code Division Multiplex Access (CDMA) technique. Its conceptual view is shown in FIG. 4.
FIG. 4 is a view for explaining the interference in performing the OFDMA. In this figure, the base stations 20 and 22 are present, and terminals 4 and 5 belong to the same sector. A terminal 3 belongs to the same base station but the adjacent sector. A terminal 2 belongs to the sector of the adjacent base station. When the terminal 4 sends the signal upstream, the base station 20 specifies beforehand the sub-channel available to the terminal 4. Also, the different sub-channel is specified for the terminal 5. Accordingly, the terminals 4 and 5 may send the signals at the same time, but use the different frequencies for use in the communication, whereby the signals sent by the two terminals do not interfere with each other. On the other hand, since the terminals 2, 3 and the terminals 4, 5 are the terminals belonging to the different sectors and cells, they may possibly use the same sub-channels as the terminals 4 and 5 in the upstream transmission to make the communication. Accordingly, the interference occurs in this case. In this way, the interference in the upstream communication does not occur between the terminals belonging to the same sector, but the interference between the terminals may occur in the different cells or sectors.
The terminal located in the cell center has a short distance to the base station for communication, and does not need to send the signal with high sending power. Also, the terminal has a long distance to the adjacent cells, and therefore has small interference with the other cells even if it sends the signal with high sending power. On the other hand, the terminal located on the cell edge has a long distance to the base station for communication, and needs to send the signal with high sending power. Also, it has a near distance to the adjacent station and great interference with the other cells.
Therefore, in a system adopting the OFDMA, the terminal near the base station has almost no influence on the interference, even if the power received by the base station is set to be slightly higher. Therefore, a method for controlling the sending power so that the received power at a receiving end of the base station may be increased in accordance with an estimated propagation loss is used (refer to 3GPP TS36.213, 5.1 Uplink power control). This is called an FTPC.
4. Interference Control with Beam Form
In JP-A-2007-243258 or 3GPP R1-081827, a method for avoiding the interference is disclosed in which the base station for beam forming changes a beam pattern depending on the frequency, and randomizes the interference occurring between the adjacent stations in the frequency domain, each terminal reports its interference situation at each frequency to the base station, and the base station performs a scheduling of frequency assignment to avoid the interference.
However, in any of the documents, selection of beam formation is realized over the given entire system band, without consideration for combination with the FFR.